Fix a linker error on x86-64 Linux when not using a version script.

If the version script produced by the libvpx build system is not
 used when linking a shared library on x86-64 Linux, the constant
 data in the subpel filters produces R_X86_64_32 relocation errors
 due to the use of wrt rip addressing instead of
 wrt rip wrt ..gotpcrel.
Instead of adding a new macro for this addressing mode, this patch
 sets the ELF visibility of these symbols to "hidden", which
 allows wrt rip addressing to work without a text relocation.
This allows building a shared library without using the provided
 build system or a separate version script.
Fixes http://code.google.com/p/webm/issues/detail?id=46

Change-Id: Ie108f9d9a4352e5af46938bf4750d2302c1b2dc2